doc: fix up some HTML issues in go_spec.html

The HTML linter 'tidy' reports:

	go_spec.html:2556: Warning: unescaped & which should be written as &
	go_spec.html:3293: Warning: unescaped & or unknown entity "&s1"
	go_spec.html:3293: Warning: unescaped & or unknown entity "&a"
	go_spec.html:3294: Warning: unescaped & or unknown entity "&s2"
	go_spec.html:3294: Warning: unescaped & or unknown entity "&a"
	go_spec.html:2045: Warning: trimming empty <p>
	go_spec.html:4526: Warning: trimming empty <ul>
	go_spec.html:4533: Warning: trimming empty <ul>
	go_spec.html:4539: Warning: trimming empty <ul>

This CL fixes all but the <ul> ones, which I think should be fixed
but are defended by a comment.

Change-Id: I0ca88f5e80755024801877ab1298025ecf8f10c5
Reviewed-on: https://go-review.googlesource.com/c/go/+/214457Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>

doc/go_spec.html

@@ -2042,7 +2042,7 @@ of the last non-empty expression list.
 <p>
 A type declaration binds an identifier, the <i>type name</i>, to a <a href="#Types">type</a>.
 Type declarations come in two forms: alias declarations and type definitions.
-<p>
+</p>
 
 <pre class="ebnf">
 TypeDecl = "type" ( TypeSpec | "(" { TypeSpec ";" } ")" ) .
@@ -2553,7 +2553,7 @@ does not have the same effect as allocating a new slice or map value with
 </p>
 
 <pre>
-p1 := &[]int{}    // p1 points to an initialized, empty slice with value []int{} and length 0
+p1 := &amp;[]int{}    // p1 points to an initialized, empty slice with value []int{} and length 0
 p2 := new([]int)  // p2 points to an uninitialized slice with value nil and length 0
 </pre>
 
@@ -3290,8 +3290,8 @@ array with the operand.
 
 <pre>
 var a [10]int
-s1 := a[3:7]   // underlying array of s1 is array a; &s1[2] == &a[5]
-s2 := s1[1:4]  // underlying array of s2 is underlying array of s1 which is array a; &s2[1] == &a[5]
+s1 := a[3:7]   // underlying array of s1 is array a; &amp;s1[2] == &amp;a[5]
+s2 := s1[1:4]  // underlying array of s2 is underlying array of s1 which is array a; &amp;s2[1] == &amp;a[5]
 s2[1] = 42     // s2[1] == s1[2] == a[5] == 42; they all refer to the same underlying array element
 </pre>